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 ###Instrução: Quantos espectadores totais têm 21 de abril de 2010 como a exibição original no canal 4?
 ###Tabelas: CREATE TABLE by_rider (by_rider_id AUTO_INCREMENT PRIMARY KEY, Rank INT, Rider VARCHAR, Country VARCHAR, Winning span VARCHAR, MotoGP/500cc INT, 350cc INT, Moto2/250cc INT, Moto3/125cc INT, 80cc/50cc INT, Total INT) CREATE TABLE first_qualifying_round (first_qualifying_round_id AUTO_INCREMENT PRIMARY KEY, Team #1 VARCHAR, Agg. VARCHAR, Team #2 VARCHAR, 1st leg VARCHAR, 2nd leg VARCHAR) CREATE TABLE subnet_and_host_counts (subnet_and_host_counts_id AUTO_INCREMENT PRIMARY KEY, Prefix size VARCHAR, Network mask VARCHAR, Available subnets INT, Usable hosts per subnet INT, Total usable hosts INT) CREATE TABLE united_kingdom_channel_4 (united_kingdom_channel_4_id AUTO_INCREMENT PRIMARY KEY, Episode number INT, Title VARCHAR, Original airing on Channel 4 VARCHAR, Time of airing on Channel 4 VARCHAR, Original airing on E4 VARCHAR, Time of airing on E4 VARCHAR, Position in Channel 4s ratings a VARCHAR, Position in E4s ratings b VARCHAR, Total viewers VARCHAR) CREATE TABLE commanders-in-chief___commanders (commanders-in-chief___commanders_id AUTO_INCREMENT PRIMARY KEY, Name VARCHAR, Branch VARCHAR, Term began VARCHAR, Term ended VARCHAR, Time in office VARCHAR)  
SELECT COUNT Total viewers FROM united_kingdom_channel_4 WHERE Original airing on Channel 4 = April 21, 2010